1. Field of the Invention
This invention relates to a method of and a structure for fixing an optical element on a mount.
2. Description of the Related Art
There have been put into practice, as a system for generating a laser beam in an ultraviolet region, a wavelength-conversion laser which converts an infrared beam radiated from a semiconductor-laser-excited solid state laser into its third harmonic in a ultraviolet region, an excimer laser, and an Ar-laser.
Recently, there have been provided GaN series semiconductor lasers (laser diodes) which generate a laser beam of a wavelength close to 400 nm as disclosed in “Japanese Applied Physics Letters, vol. 37 (1998), p.L1020”.
Light sources generating a laser beam in such a wavelength range can be employed as an exposure light source in an exposure system for exposing a photosensitive material which is sensitive to light in a predetermined wavelength range including an ultraviolet region of 350 to 420 nm (will be referred to as “the ultraviolet region”, hereinbelow). In this case, the exposure light source is, naturally, required to have an output power sufficient to sensitize the photosensitive material.
However, the excimer laser is disadvantageous in that the system is too large in size and too high in cost and maintenance cost.
Further, when the wavelength-conversion laser which converts an infrared beam into its third harmonic in a ultraviolet region is employed, it is very difficult to obtain a high output power due to very low wavelength conversion efficiency of the wavelength-conversion laser. At present, the practical level is on a level such that a fundamental wave (1064 nm) at 10 W is oscillated by a laser diode at 30 W and converted to its second harmonic (532 nm) at 3 W and a third harmonic (355 nm) which is at a frequency equal to the sum frequency of the fundamental wave and its second harmonic is obtained at 1 W. The electro-optic efficiency of the laser diode is about 50% and the conversion efficiency to an ultraviolet beam is as very low as 1.7%. Further since employing an expensive wavelength conversion element, such a wavelength-conversion laser is substantially high in cost.
Further, the Ar-laser is very low as 0.005% in electro-optic efficiency and is very short as about 1000 hours in service life.
Further, in the GaN series laser diodes, since a GaN crystal substrate of a low dislocation cannot be obtained, attempts of obtaining a high output power and a high reliability have been made by forming a low dislocation region of about 5 μm by a method of growth generally called ELOG and forming a laser region on the low dislocation region. However, even in GaN series laser diodes produced in such a manner, those that are as high as 500 mW to 1 W in output power have not been commercialized due to a difficulty of making a substrate of low dislocation.
As another attempt of increasing the output power of the laser diode, there can be conceived a method involving obtaining, for instance, a laser beam at 10 W by forming one hundred cavities each outputs a laser beam at 100 mW. However, it is almost not practical to make as many as one hundred cavities at a high yield. It is especially true of GaN series laser diodes where it is difficult to raise the yield to 99% or higher even the diodes have only a single cavity.
In view of the foregoing observations, this applicant has proposed a laser system which can generate a laser beam at an especially high output power. (U.S. Pat. No. 6,718,088: will be simply referred to as “patent publication 1”, hereinbelow) The laser system disclosed in patent publication 1 comprises a plurality of laser diodes, a single multi-mode optical fiber, and a collective optical system which collects laser beams radiated from the laser diodes and couples the collected laser beams to the multi-mode optical fiber. In one preferred embodiment of the laser system, the laser diodes are disposed so that their light emitting points are arranged in one direction.
In patent publication 1, there is also disclosed a multi-cavity laser diode having a plurality of light emitting points. One or more such multi-cavity laser diodes can be used in place of the laser diodes in the laser system disclosed in patent publication 1.
In the laser system where a plurality of laser beams are synthesized together, the laser diodes are generally fixed to a block such as a heat radiating block of Cu, Cu alloy or AlN.
Since the laser beam is radiated from each light emitting point of a laser diode having one or more light emitting points in the form of divergent light, it is necessary to first pass each of the laser beams through a collimator lens and then pass the collimated laser beams through a collective lens in order to converge the laser beams on one point. Though the collimator lenses may be disposed separately from each other, the laser system can be small in size and can be easily adjusted when the collimator lenses are integrated into a collimator lens array in which a plurality of collimator lenses are arranged in a line.
When a collimator lens array is employed, each collimator lens of the array must be positioned so that its optical axis is accurately aligned with the light emitting axis of the corresponding laser diode. Otherwise, a plurality of laser beams cannot be converged in a small spot, and accordingly, for instance, it becomes unfeasible to finely imagewise expose the above described photosensitive material with the laser beams.
The laser diodes and/or the collimator lens array are generally fixed to a flat mount by solder. Conventionally, the laser diode or the collimator lens array is fixed in place by positioning the laser diode or the collimator lens array on molten solder which has been melt on the mount and solidifying the molten solder. In Japanese Patent Application 2002-287640 filed by this applicant, an improved example of such a fixing method is disclosed.
However, such a method of fixing an optical element gives rise to a problem that it is difficult to stably hold the optical element such as the laser diode or the collimator lens array on the solder the state of which changes while it is solidified and the optical element can be slightly shifted from the correct position when the solder is solidified. In this case, accuracy of positioning the laser diode and the collimator lens array with respect to each other deteriorates. For example, it becomes difficult to secure a positioning accuracy of 0.5 μm. If the accuracy of positioning the laser diode and the collimator lens array with respect to each other deteriorates, more laser beams impinge upon the corresponding collimator lenses in the collimator lens array with their optical axes out of alignment with the optical axes of the corresponding collimator lenses as the number of the laser beams to be synthesized is increased, whereby it becomes difficult to obtain a high intensity synthesized laser beam.
Though a problem caused in positioning and fixing a laser diode and a collimator lens array has been described by way of example, there has been a wide demand that optical elements other than the laser diode or the collimator lens array should be accurately positioned and fixed when the optical elements are to be fixed on a flat mount by cement such as solder.